Electric vehicles are known in the art and use one or more electric motors or traction motors for propulsion. (A traction motor is an electric motor that is used for propulsion of a vehicle.) Typically, electrical energy stored in on-board energy storage units, such as batteries and ultracapacitors, is fed to an inverter where direct current (DC) is converted to alternating current (AC), which is then fed to multi-phase (typically 3-phase) AC motors that drive the wheels of the vehicle. Electrical propulsion is designed and often used for urban package delivery vehicles (PDV), urban transit buses, and material transport vehicles, including underground mining vehicles, where emissions also must be kept to a minimum in the enclosed, underground spaces of the environment in which these vehicles operate. In connection with this, vehicles with electrical propulsion, as opposed to utilizing internal combustion engines, are often utilized to improve air quality, increase productivity, and reduce the cost of a mining operation, plus emissions for package delivery, and public transport in urban areas.
Thermal control of the energy storage units in underground mining electric vehicles is often challenging since the energy storage unit on-board the vehicle (e.g., the battery or ultracapacitor) is often required to operate over a specified temperature range. If, for example, the temperature is too high, the life of the battery or other energy storage unit can be significantly reduced. In addition, if the energy storage unit operates at significantly elevated temperatures, such thermal conditions may also lead to potential safety issues.
Conversely, depending on the particular battery technology and chemistry, if the operating temperature is too low, performance of the battery can degrade. Or, if for example, a high temperature battery such as a sodium metal halide chemistry battery is utilized, the battery may not function properly when the operating temperature range is below 250° C. In addition, operation at the low end of the operating temperature range can cause low efficiency and reduced power capability. Likewise, operation of the battery at elevated temperatures above prescribed temperature levels may shorten the battery's life, cause faults, and the like.
FIG. 1 illustrates a prior art propulsion system 10 for a vehicle. As shown therein, the system includes a single energy storage unit 12, an inverter 14, at least one motor 16 and a control unit 18 for controlling the distribution of electrical energy from the energy storage unit 12, as discussed in detail hereinafter. The storage unit 12 may be a battery having of a type and chemistry known in the art, and is configured to store electrical energy from an external power station, from stored electricity originally form an external power source, from an on-board electrical generator, or during capture of regenerative braking energy while the vehicle is decelerating or holding speed on a down-hill grade. The energy storage unit 12 is configured to feed stored electrical energy, where energy is defined as power for a specified time (i.e. Whr), in the form of DC current at a particular voltage, to the inverter 14, where it is converted to alternating current electrical excitation. From the inverter 14, the alternating current and voltage electrical excitation is fed to the motor 16 which drives the wheels of a vehicle 20 on which the system 10 is deployed. As will be readily appreciated, however, all of the power and energy for propelling the vehicle employing the propulsion system 10 must be provided by the single energy storage unit 12.
In view of the above, there is a need for an energy management system and method for electric vehicles that alleviates thermal issues related to battery operation.